Adaptive time diversity and spatial diversity for OFDM

ABSTRACT

An adaptable orthogonal frequency-division multiplexing system (OFDM) that uses a multiple input multiple output (MIMO) to having OFDM signals transmitted either in accordance with time diversity to reducing signal fading or in accordance with spatial diversity to increase the data rate. Sub-carriers are classified for spatial diversity transmission or for time diversity transmission based on the result of a comparison between threshold values and at least one of three criteria. The criteria includes a calculation of a smallest eigen value of a frequency channel response matrix and a smallest element of a diagonal of the matrix and a ratio of the largest and smallest eigen values of the matrix.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

Reference is made copending patent application entitled: CHANNELSESTIMATION FOR MULTIPLE INPUT—MULTIPLE OUTPUT, ORTHOGONAL FREQUENCYDIVISION MULTIPLEXING (OFDM) SYSTEM, U.S. patent application Ser. No.09/751,166 and whose contents are incorporated by reference with respectto channels estimation. This application is a utility patent applicationbased on provisional patent application Ser. No. 60/229,972, filed Sep.1, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to adapting time diversity and spatial diversityfor use in an orthogonal frequency-division multiplexing (OFDM)environment, using a multiple input and multiple output (MIMO)structure.

2. Discussion of Related Art

A multiple input, multiple output (MIMO) structure has multiplecommunication channels that are used between transmitters and receivers.A space time transmitter diversity (STTD) system may be used on a MIMOstructure, but it will not increase the data throughput. Indeed, for ahigh level configuration, the data rate may even reduce. In an STTDsystem, the transmitters deliver the same information content withinconsecutive symbol duration so that time diversity may be exploited. Toefficiently use the multiple transmitters of the MIMO structure,however, the transmission data rate needs to be increased.

The most straightforward solution to increase the transmission data rateis to in forward error correction (FEC) dump independent data to eachtransmitter. A forward error correction (FEC) encoder produces in-phaseand quadrature-phase data streams for the digital QAM modulator inaccordance with a predetermined QAM constellation. The QAM modulator mayperform baseband filtering, digital interpolation and quadratureamplitude modulation. The output of the QAM modulator is a digitalintermediate frequency signal. A digital to analog (D/A) convertertransforms the digital IF signal to analog for transmission.

The problem arises, however, as to how to safely recover the transmitteddata. For a 2×2 system (two transmitters, two receivers) for example,after the channel information is obtained, the recovery process entailsformulating two equations with two unknowns that need to be solved. Thetwo unknowns may be determined only if the 2×2 channel is invertible. Inpractice, however, two situations may be encountered, i.e., the channelmatrix is rank deficient so the unknowns cannot be determined or thefrequency response channel matrix is invertible but has a very smalleigen value.

The first situation arises when the channels are highly correlated,which may be caused either by not enough separation of the transmittersor by homology of the surroundings. For the second situation, althoughthe equations are solvable, the solution can cause a high bit error rate(BER), because a scale up of the noise can result in an incorrectconstellation point.

Orthogonal frequency-domain multiplexing (OFDM) systems were designedconventionally for either time diversity or for space diversity, but notboth. The former will provide a robust system that combats signal fadingbut cannot increase the data rate capacity, while the latter canincrease the data rate capacity but loses the system robustness. An OFDMsignal contains OFDM symbols, which are constituted by a set ofsub-carriers and transmitted for a fixed duration.

The MIMO structure may be used for carrying out time diversity for anOFDM system. For instance, when one transmitter transmits an OFDMsignal, another transmitter will transmit a fully correlated OFDM signalto that transmitted by the one transmitter. The same OFDM signal istransmitted with, for instance, a fixed OFDM duration.

On the other hand, spatial diversity entails transmitting independentsignals from different transmitters. Thus, transmitting two independentOFDM signals from two transmitters, respectively, results in a doubledata rate capacity from the parallel transmission that occurs.

When the signal to noise ratio (SNR) is low, the frame error rate (FER)is large, so that a data packet transmission will be decoded incorrectlyand will need to be retransmitted. The quality of service (QoS) definesthe number of times that the same packet can be retransmitted, eg.,within an OFDM architecture. The OFDM system on a MIMO structure,therefore, should be adaptable to ensure that the QoS is maintained.

For any given modulation and code rate, the SNR must exceed a certainthreshold to ensure that a data packet will be decoded correctly. Whenthe SNR is less than that certain threshold, the bit error rate (BER)will be larger, which results in a larger FER. The larger the FER, themore retransmissions of the same packet will be required until thepacket is decoded correctly. Thus, steps may need to be taken to providethe OFDM system with a higher gain. If the SNR is at or above thethreshold, then there is no need to increase the gain of thearchitecture to decode the data packets correctly. One challenge is toadapt the OFDM system to use time diversity when signal fading isdetected as problematic and to use spatial diversity at other time toincrease the data rate transfer.

In a conventional OFDM system, there are many OFDM modes, for examplesare the 1k mode (1024 tones) and the half k mode(512 tones). For 1kmode, the number of sub-carriers is 1024 and for the half k mode, thenumber of sub-carriers is 512. The 1k mode is suitable for a channelwith long delay and slow temporal fading, while the 512 mode is suitablefor the channel with a short delay and fast temporal fading. But whichmode will be used is really depending on the real environment.

A transaction unit of a conventional OFDM signal is an OFDM frame thatlasts 10 ms. Each OFDM frame consists of 8 OFDM slots and each slotlasts 1.25 ms. Each OFDM slot consists of 8 OFDM symbols and some of theOFDM symbols will be the known preambles for access and channelsestimation purposes. An OFDM super frame is made up of 8 OFDM frames andlasts 80 ms.

In addition to transmitted data, an OFDM frame contains a preamble,continual pilot sub-carriers, and transmission parametersub-carriers/scattered sub-carriers. The preamble contains OFDM symbolsthat all used for training to realize timing, frequency and samplingclock synchronization acquisitions, channel estimation and a C/Icalculation for different access points.

The continual pilot sub-carriers contain training symbols that areconstant for all OFDM symbols. They are used for tracking the remainingfrequency/sampling clock offset after the initial training.

The transmission parameter sub-carriers/scattered sub-carriers arededicated in each OFDM symbol and reserved for signaling of transmissionparameters, which are related to the transmission scheme, such aschannel coding, modulation, guarding interval and power control. Thetransmission parameter sub-carriers are well protected and therefore canbe used as scattered pilot sub-carriers after decoding.

One application for determining whether sub-carriers should be assignedto time diversity or spatial diversity is to conform with statisticalanalysis of traffic demands during particular times of the day, such aspeak and off-peak. The OFDM system may preferably bias toward eithertime diversity or spatial diversity based on such a statisticalanalysis.

BRIEF SUMMARY OF THE INVENTION

One aspect of the invention pertains to employing adaptive STDD andspatial multiplexing (SM) based on comparing the channel condition ofeach sub-carrier with a threshold. When a sub-carrier is accommodated onchannels that have a “well conditioned” channel matrix, spatialmultiplexing may be used to create independent transmission paths andtherefore increase the data rate. A “well conditioned” channel matrixarises when the smallest eigen value is not too small as compared to athreshold value, such as the noise power increase when multiplied by itsinverse. For those sub-carriers whose channel matrices have smallereigen values, the receiver cannot recover the parallel transmittedinformation symbols. As a result, STTD is used to guarantee a robustsystem.

Encoders associated with the transmitter side encode or classifysub-carriers in accordance with one of two groups based on a feedbacksignal; one of the groups is to forward error correction (FEC) timediversity and the other of the two groups is to forward error correction(FEC) spatial diversity. This grouping is based on results from acomparison made at the receiver side between a threshold value andeither a calculated smallest eigen value of a frequency response matrix,the smallest element in a diagonal of the matrix, or a ratio of thelargest and smallest eigen values in the matrix.

The threshold value is based on the transmitter and receiver antennaconfiguration, environmental constraints of the OFDM communicationsystem, and/or on statistical analysis of communication traffic demands.The estimate value is derived from channel estimation of multiplechannels of multi-input multi-output (MIMO) type systems.

Time diversity is used to reduce adverse signal fading. Spatialdiversity is used to increase the data rate, which time diversity cannotdo. When sub-carriers use time diversity, it means that signal fading isstrong so that parallel transmission of data packets can not be done toovercome the insufficient gain problem. Instead, time diversity is usedto get the necessary gain for the OFDM system, even though the data ratecapacity suffers. An SNR gain is assured with time diversity, because ofthe orthogonality matrix pattern inherent among transmitted samples inthe OFDM system. On the other hand, when sub-carriers use spatialdiversity, signal fading is weak so that parallel transmissions mayoccur to increase the data rate capacity. Thus, there is no need toincrease the gain of the OFDM system, which means that the data rate maybe increased.

In operation, the OFDM system of the invention may start transmission ofdata packets with either time diversity or spatial diversity. Thereceiver side will estimate the channels and decode the data packets.After the channel information, is obtained, the receiver side willcalculate the eigen values of the channel matrices to the extentpossible. The controller then determines whether the sub-carrier to usetime diversity or spatial diversity based on one of three criteria (onlyone of which is dependent upon the eigen value calculation). Thereceiver then reports back or feedbacks to the transmitter side withthis information, i.e., about whether the sub-carrier is to use timediversity or spatial diversity so as to trigger the next round oftransmission accordingly.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

For a better understanding of the present invention, reference is madeto the following description and accompanying drawings, while the scopeof the invention is set forth in the appended claims.

FIG. 1 is a schematic representation of a generic multi-input,multi-output orthogonal frequency-division multiplexing transmitter inaccordance with an embodiment of the invention.

FIG. 2 is a schematic representation of an orthogonal frequency-divisionmultiplexing symbol.

FIG. 3 is a space time transmitter diversity (STTD) orthogonalfrequency-division multiplexing (OFDM) encoder for loading data to asub-carrier in G1 which will be specified in the forthcoming sections.

FIG. 4 is a spatial multiplexing (SM) orthogonal frequency-divisionmultiplexing (OFDM) encoder for loading data to a sub-carrier in G2which will be specified in the forthcoming sections.

FIG. 5 is a schematic representation of two pure STTD transmitters thatsave one half of the IFFT computation.

FIG. 6 is a schematic representation of four pure STTD transmitters thatsave three fourths of the IFFT computation.

FIG. 7 is a schematic representation a generic receiver structure.

FIG. 8 is a schematic representation of configurations of a two receiverantenna case and a three receiver antenna case.

DETAILED DESCRIPTION OF THE INVENTION

The invention concerns a practical time and spatial diversitycombination that fits into an OFDM system. The OFDM system of theinvention can automatically adapt the channel variation and make tradeoff between time diversity and spatial diversity. In an exemplaryenvironment, the data rate can be increased 1.8 times for 2×2configuration (2 transmitters, 2 receivers), which gives 80 Mbps, and2.7 times for 3×3 configuration) 3 transmitters, 3 receivers) whichgives 121 Mbps within 6 MHz, while keep the robustness of the system.

Turning to the drawing, FIG. 1 shows a generic MIMO and OFDM transmittersystem. In the figure, STTD and SM are the abbreviations ofSpace-Time-Transmitter Diversity and Spatial Multiplexing. The MIMO OFDMis configured as two level adaptations as shown in FIG. 1, namely,space/time diversity adaptation and coding/modulation adaptation. Thespace/time diversity adaptation is determined by the carrier tointerference power ratio or signal to noise power ratio.

Information data is fed into adaptive coding modulation, the modulationis multiplexed and fed into adaptive space/time diversity encoding andassignment. A receiver feedback to provide feedback signals to theadaptive coding of modulation, multiplexer and adaptive space/timediversity is also provided. The multiplexed signals in the adaptivespace/time diversity pass through STTD/SM OFDM encoders and the encodedsignals transmit to associated antennas. The adaptive coding andmodulation includes a forward error correction (FEC) encoder, aninterleaver and an m-PSK modular.

If x MHz bandwidth is available, then Orthogonal Frequency DivisionMultiplexing OFDM is to chop this whole spectrum into many small piecesof equal width and each of them will be used as a carrier. The width ofthe piece will be determined by delay spread of the targetedenvironment.

The STTD/OFDM encoder is responsible for the assignment of theconstellation points to each sub-carrier. For M transmitters, M OFDMsymbols data are loaded in general (so the bit loading will becalculated according to this number), but it will depend on the STTDstructure. FIG. 2 illustrates one OFDM symbol.

For each sub-carrier that is indexed k, its loading will be determinedby its corresponding channel condition. For N receivers, the frequencychannel responses may be represented by an M×N matrix, say H(k). Thechannel condition will be described by one of the following 3 criteria.

-   1. Smallest eigen value of H(k)H(k)*-   2. Smallest element of the diagonal of H(k)H(k)*-   3. The ratio of largest and smallest eigen values of H(k)H(k)*

A set of thresholds for each criterion and for each system configurationis used. These thresholds will be service parameters and can be used asquality of service (QoS) or billing purposes.

With each criterion and a given threshold, all the sub-carriers will beclassified into two groups G1 and G2 by a controller at the receiverside. The controller directs the transmission of a feedback signalindicative of the result of the classification. The feedback signal isreceived at the transmitter side and interpreted by a controller at thetransmitter side. The sub-carriers classified in G1 will use STTDencoder at the transmission side while those classified in G2 will usethe SM encoder at the transmission side.

After the subcarriers have been classified into the two groups G1 andG2, the modulation scheme on each sub-carrier will be determined by theestimated C/I (carrier to interference ratio) or SNR (signal to noiseratio). As a result, a modulation scheme, such as of QPSK or m-PSK orvarious QAM, will be selected to satisfy QoS (quality of service) basedon the determination made by the estimated C/I or SNR. This is anotherlevel adaptation that may maximize the throughput gain.

For instance, when the QoS is defined, the FER (frame error rate) may beten percent. The goal is to choose a modulation scheme according to theperceived C/I or SNR to satisfy this QoS, yet still maximizing thethroughput of data flow. To achieve this, a pre-defined look-up tablemay be accessed that is in accordance with various QoS.

In determining which modulation scheme will satisfy the criteria, theC/I or SNR estimation is done during mobile access, after looking forthe strongest signal from the base station first. Based on suchknowledge and estimation, one is able to get a rough idea as to whichmodulation scheme should be used. Regardless of the modulation schemeselected initially, the invention is configured to automatically adapttoward whichever modulation scheme represents the optimal modulation.

FIG. 3 shows how to load data on sub-carrier k for a situation involving2 transmitters for example. This data loading is done within a pair ofOFDM symbols. As can be appreciated, apparently one sample has beentransmitted twice within 2 OFDM symbols duration via 2 transmitters.Thus, the data rate is the same as for the one transmitter OFDM system.

FIG. 4 shows how to load data on sub-carrier k in G2 for a situationinvolving 2 transmitters.

In this case, each transmitter transmits independent data and thereforethe data rate is double for 2 transmitters and M times for Mtransmitters.

The adaptive time diversity and spatial diversity for OFDM works asfollows. Starting out, an STTD mode is used for all sub-carriers. Thereceiver estimates the channel profiles and then directs a feedback ofits preference either to STTD or spatial multiplexing (SM) on eachsub-carrier.

The whole sub-carrier indices {K_(min), K_(min)+1, . . . , K_(max)} arethen divided into two disjoint subsets I_(sttd) and I_(sm). The one withfewer elements will be the feedback to the transmitters. The extremecase is that one of them is an empty set, which means use of either pureSTTD or pure SM. As in the pure STTD system, the transmitters alwaysconsider two OFDM symbols as the basic transmission unit for 2×2configuration and M OFDM symbols for a system has M transmitters.

The number of input bits, however, needs to be calculated according to amodulation scheme and a dynamic distribution of I_(sttd) and I_(sm).More precisely, the number of bits needed for the two consecutive OFDMsymbols is 2×|I_(sttd)|L+4×|I_(sm)|L, where L is the modulation levelwhich equals to 2, 3, 4 5, 6, 7, 8.

When a granularity problem arises, the two OFDM symbols are repacked tofit the granularity by removing some sub-carriers from I_(sm) intoI_(sttd). This may sacrifice the data rate somewhat, but keep the systemrobust.

In the receiver side, a quadrature amplitude modulation QAM de-mappingblock is used to de-map the received data according to I_(sttd) andI_(sm).

STTD is the baseline of the service quality. This means that whenparallel transmission is carried out in the designated communicationchannels, then it is guaranteed parallel transmission, because the BERor FER will be controlled to achieve the necessary QoS. The transmitterswill propagate the transmissions at the same constant power and themodulation will be the same for each transmitter. Thus, no power pouringtechnique needs to be employed.

Three thresholds are used to classify the sub-carriers. Indeed, thethreshold can be used as a service parameter and tuned aggressive toeither STTD mode or SM mode according to customer demand, i.e., based onstatistical analysis of that demand.

As an example, for the case where the smallest eigen value is used asthe threshold in a 2×2 configuration (2 transmitters, 2 receivers),there is a 60% opportunity to do parallel transmission with 0.5 as thethreshold value, which may be scale the noise 3 dB up. for a 2×4configuration (2 transmitters, 4 receivers), there is an 80% opportunityto do parallel transmission with 1 as the threshold value, which mayeven reduce the noise.

FIG. 5 shows a special, but very practical situation, which shows twopure STTD transmitters that save ½ of an inverse fast Fourier transform(IFFT) computation. The present invention may automatically switch tothis scenario in a vulnerable environment involving 2 transmitters.

Conventionally, one would expect each transmitter to transmit 2 OFDMsymbols every 2 OFDM symbol duration. Thus, there are 4 OFDM symbolstransmitted for every 2 OFDM duration that go through a respectiveindependent IFFT computation engine. This means that a complex numberIFFT computation is expected to be conducted four times.

For a pure STTD implementation with 2 and 4 transmit antennas, thecomputational efficient implementation is shown in FIGS. 5 and 6respectively. The scheme in FIG. 5 requires ½ of the IFFT computationand the scheme in FIG. 6 requires ¼ of the IFFT computation as comparedwith a straightforward implementation that performs the computationsseparately.

In accordance with FIG. 5, however, there is data crossing between twotransmitters, which saves two IFFT computations. Yet, it provides fourIFFT outputs, which is exactly the same results where four independentIFFTs are used. Although four IFFT operations are shown in FIG. 5, theyare operating on real vectors, which means the computational complexityof a real IFFT equals the complex IFFT with a half size. Therefore, thecomputational time saving comes from the relationship between IFFT on avector and its conjugate.

In FIG. 5, the bits are coded bits, which are the input to variableM-PSK/QAM mapping. The mapping will map the bits to the correspondingconstellation points according to the Gray rule; constellation pointshere refer to any modulation scheme, such as QPSK, m-PSK, QAM, etc. Theconstellation vector will be inserted with a pilot into a multiplex andthen into first in first out (FIFO) buffers.

The designations S₀, S₁, S₂, S₃, S₂₀₄₆, S₂₀₄₇, in the FIFO bufferrepresent complex vectors. The function Re{} refers to just taking thereal part of the complex vector. The designation Im{} refers to justtaking the imaginary part of the complex vector. The real and imaginaryparts are fed as input into IFFTs. The designation D/A refers to adigital to analog converter.

The transmission order for the first transmitter is OFDM symbol b andthen d . . . ; the transmission order for the second transmitter is OFDMsymbol g and then f etc. Before each OFDM symbol is transmitted, thecyclic extension will be appended somewhere in the OFDM symbol.

Periodically inserted preambles will serve for the timing recovery,framing, frequency offset estimation, clock correction and overallchannel estimation The estimated channel samples will be used for thecontinuous spectrum channel reconstruction. Pilot symbols will serve forphase correction, final tuning of channel estimation.The mathematical equivalence for FIG. 5 is as follows.${b = {I\; F\; F\;{T\begin{bmatrix}S_{0} \\S_{2} \\\vdots \\S_{2046}\end{bmatrix}}}},{d = {I\; F\; F\;{T\begin{bmatrix}{- S_{1}^{*}} \\{- S_{3}^{*}} \\\vdots \\{- S_{2047}^{*}}\end{bmatrix}}}},{f = {I\; F\; F\;{T\begin{bmatrix}S_{1} \\S_{3} \\\vdots \\S_{2047}\end{bmatrix}}}},{g = {I\; F\; F\;{T\begin{bmatrix}S_{0}^{*} \\S_{2}^{*} \\\vdots \\S_{2046}^{*}\end{bmatrix}}}}$

FIG. 6 shows four Pure STTD Transmitters that represents a rate ¾ STTDencoder as:

Tx1 S(0) −S(1)* S(2)*/sqrt(2) S(2)/sqrt(2) Tx2 S(1) S(0)* S(2)*/sqrt(2)−S(2)/sqrt(2) Tx3 S(2)/sqrt(2) S(2)/sqrt(2) −Re{S(0)} + −Re{S(1)} +jIm{S(1)} jIm{S(0) Tx4 S(2)/sqrt(2) S(2)/sqrt(2) Re{S(1)} + −Re{S(0)} −jIm{S(0)} jIm{S(1) Time [0 T] [T 2T] [2T 3T] [3T 4T]Such an STTD encoder encodes every 3 OFDM symbols into 4 OFDM symbolsand transmits to 4 antennas. FIG. 6 scheme requires ¼ IFFT computationcompared to the straightforward implementation. The reason whycomputation is saved is for the same reasons as in FIG. 5. Theparameters there are defined respectively as follows:${b = {I\; F\; F\;{T\begin{bmatrix}S_{0} \\S_{3} \\\vdots \\S_{3069}\end{bmatrix}}}},{g = {I\; F\; F\;{T\begin{bmatrix}S_{0}^{*} \\S_{3}^{*} \\\vdots \\S_{3069}^{*}\end{bmatrix}}}}$ ${f = {I\; F\; F\;{T\begin{bmatrix}S_{1} \\S_{4} \\\vdots \\S_{3070}\end{bmatrix}}}},{d = {I\; F\; F\;{T\begin{bmatrix}S_{1}^{*} \\S_{4}^{*} \\\vdots \\S_{3070}^{*}\end{bmatrix}}}}$ ${q = {I\; F\; F\;{T\begin{bmatrix}S_{2} \\S_{5} \\\vdots \\S_{3071}\end{bmatrix}}}},{u = {I\; F\; F\;{T\begin{bmatrix}S_{2}^{*} \\S_{5}^{*} \\\vdots \\S_{3071}^{*}\end{bmatrix}}}}$

FIG. 7 is an abstract diagram of a generic receiver structure.

STTD/SM OFDM decoder is sub-carrier based decoder. The structure andconfiguration of the STTD/SM OFDM decoder will depend on thearchitecture configuration.

Suppose sub-carrier m is STTD coded, i.e. m belongs to G1.

For a 2×2 configuration:S(2m) and S(2m+1) are decoded by solving the following equations$\begin{bmatrix}{y_{1}\left( {q,m} \right)} \\{y_{1}\left( {{q + 1},m} \right)}^{*}\end{bmatrix} = {{\begin{bmatrix}{h_{11}\left( {q,m} \right)} & {h_{21}\left( {q,m} \right)} \\{h_{21}\left( {q,m} \right)}^{*} & {- {h_{11}\left( {q,m} \right)}^{*}}\end{bmatrix}\;\left\lbrack \begin{matrix}{s\left( {2m} \right)} \\{s\left( {{2m} + 1} \right)}\end{matrix} \right\rbrack} + {\quad\begin{bmatrix}{n_{1}\left( {q,m} \right)} \\{n_{1}\left( {{q + 1},m} \right)}\end{bmatrix}}}$

The assumption here is that the even indexed sample S(2m) is transmittedin qth OFDM and the odd indexed sample S(2m+1) is transmitted in (q+1)thOFDM symbol.

There are 4 equations and two unknowns. So a least mean square solutioncan be obtained by multiplying the coefficient matrix to the receiveddata vector. With the above two pairs, we will get two estimated of thesame pair of samples. Their average will be the output of the decoder.

More statistics are performed after regrouping the equations. In fact,every pair of the equations will result a solution, every 3 equationsalso provide a new estimation, and all the equations will give asolution too. There are 10 combinations in total and therefore 10estimation with these 4 equations. Their average or partial average willbe used as the solution.

A 2×3 configuration is similar to 2×2, involving 6 equations:$\begin{bmatrix}{y_{1}\left( {q,m} \right)} \\{y_{1}\left( {{q + 1},m} \right)}^{*}\end{bmatrix} = {{\begin{bmatrix}{h_{11}\left( {q,m} \right)} & {h_{21}\left( {q,m} \right)} \\{h_{21}\left( {q,m} \right)}^{*} & {- {h_{11}\left( {q,m} \right)}^{*}}\end{bmatrix}\;\left\lbrack \begin{matrix}{s\left( {2m} \right)} \\{s\left( {{2m} + 1} \right)}\end{matrix} \right\rbrack} + {\quad{{\begin{bmatrix}{n_{1}\left( {q,m} \right)} \\{n_{1}\left( {{q + 1},m} \right)}\end{bmatrix}\begin{bmatrix}{y_{2}\left( {q,m} \right)} \\{y_{2}\left( {{q + 1},m} \right)}^{*}\end{bmatrix}} = {{\begin{bmatrix}{h_{12}\left( {q,m} \right)} & {h_{22}\left( {q,m} \right)} \\{h_{22}\left( {q,m} \right)}^{*} & {- {h_{12}\left( {q,m} \right)}^{*}}\end{bmatrix}\;\left\lbrack \begin{matrix}{s\left( {2m} \right)} \\{s\left( {{2m} + 1} \right)}\end{matrix} \right\rbrack} + {\quad{{\begin{bmatrix}{n_{2}\left( {q,m} \right)} \\{n_{2}\left( {{q + 1},m} \right)}\end{bmatrix}\begin{bmatrix}{y_{3}\left( {q,m} \right)} \\{y_{3}\left( {{q + 1},m} \right)}^{*}\end{bmatrix}} = {{\begin{bmatrix}{h_{13}\left( {q,m} \right)} & {h_{23}\left( {q,m} \right)} \\{h_{23}\left( {q,m} \right)}^{*} & {- {h_{13}\left( {q,m} \right)}^{*}}\end{bmatrix}\;\left\lbrack \begin{matrix}{s\left( {2m} \right)} \\{s\left( {{2m} + 1} \right)}\end{matrix} \right\rbrack} + {\quad\begin{bmatrix}{n_{3}\left( {q,m} \right)} \\{n_{3}\left( {{q + 1},m} \right)}\end{bmatrix}}}}}}}}}$

For a 2×4 configuration, there are 8 equations: $\begin{bmatrix}{y_{1}\left( {q,m} \right)} \\{y_{1}\left( {{q + 1},m} \right)}^{*}\end{bmatrix} = {{\begin{bmatrix}{h_{11}\left( {q,m} \right)} & {h_{21}\left( {q,m} \right)} \\{h_{21}\left( {q,m} \right)}^{*} & {- {h_{11}\left( {q,m} \right)}^{*}}\end{bmatrix}\;\left\lbrack \begin{matrix}{s\left( {2m} \right)} \\{s\left( {{2m} + 1} \right)}\end{matrix} \right\rbrack} + {\quad{{\begin{bmatrix}{n_{1}\left( {q,m} \right)} \\{n_{1}\left( {{q + 1},m} \right)}\end{bmatrix}\begin{bmatrix}{y_{2}\left( {q,m} \right)} \\{y_{2}\left( {{q + 1},m} \right)}^{*}\end{bmatrix}} = {{\begin{bmatrix}{h_{12}\left( {q,m} \right)} & {h_{22}\left( {q,m} \right)} \\{h_{22}\left( {q,m} \right)}^{*} & {- {h_{12}\left( {q,m} \right)}^{*}}\end{bmatrix}\;\left\lbrack \begin{matrix}{s\left( {2m} \right)} \\{s\left( {{2m} + 1} \right)}\end{matrix} \right\rbrack} + {\quad{{\begin{bmatrix}{z_{2}\left( {q,m} \right)} \\{z_{2}\left( {{q + 1},m} \right)}\end{bmatrix}\begin{bmatrix}{y_{3}\left( {q,m} \right)} \\{y_{3}\left( {{q + 1},m} \right)}^{*}\end{bmatrix}} = {{\begin{bmatrix}{h_{13}\left( {q,m} \right)} & {h_{23}\left( {q,m} \right)} \\{h_{23}\left( {q,m} \right)}^{*} & {- {h_{13}\left( {q,m} \right)}^{*}}\end{bmatrix}\;\left\lbrack \begin{matrix}{s\left( {2m} \right)} \\{s\left( {{2m} + 1} \right)}\end{matrix} \right\rbrack} + {\quad{{\begin{bmatrix}{n_{3}\left( {q,m} \right)} \\{n_{3}\left( {{q + 1},m} \right)}\end{bmatrix}\begin{bmatrix}{y_{4}\left( {q,m} \right)} \\{y_{4}\left( {{q + 1},m} \right)}^{*}\end{bmatrix}} = {{\begin{bmatrix}{h_{14}\left( {q,m} \right)} & {h_{24}\left( {q,m} \right)} \\{h_{24}\left( {q,m} \right)}^{*} & {- {h_{14}\left( {q,m} \right)}^{*}}\end{bmatrix}\;\left\lbrack \begin{matrix}{s\left( {2m} \right)} \\{s\left( {{2m} + 1} \right)}\end{matrix} \right\rbrack} + {\quad\begin{bmatrix}{n_{4}\left( {q,m} \right)} \\{n_{4}\left( {{q + 1},m} \right)}\end{bmatrix}}}}}}}}}}}}$

For a 4×2 configuration, there are 8 equations and 3 unknowns$\begin{bmatrix}{y_{1}\left( {q,m} \right)} \\{y_{1}\left( {{q + 1},m} \right)} \\{y_{1}\left( {{q + 2},m} \right)} \\{y_{1}\left( {{q + 3},m} \right)}\end{bmatrix} = {\begin{bmatrix}\left. {s\left( {{3m} - 3} \right)} \right) & {s\left( {{3m} - 2} \right)} & \frac{s\left( {{3m} - 1} \right)}{\sqrt{2}} & \frac{s\left( {{3m} - 1} \right)}{\sqrt{2}} \\{- {s\left( {{3m} - 2} \right)}^{*}} & {s\left( {{3m} - 3} \right)}^{*} & \frac{s\left( {{3m} - 1} \right)}{\sqrt{2}} & {- \frac{s\left( {{3m} - 1} \right)}{\sqrt{2}}} \\\frac{{s\left( {{3m} - 1} \right)}^{*}}{\sqrt{2}} & \frac{{s\left( {{3m} - 1} \right)}^{*}}{\sqrt{2}} & {\eta(m)} & {\kappa(m)} \\\frac{{s\left( {{3m} - 1} \right)}^{*}}{\sqrt{2}} & {- \frac{{s\left( {{3m} - 1} \right)}^{*}}{\sqrt{2}}} & {\nu(m)} & {\zeta(m)}\end{bmatrix}{\quad\;{\left\lbrack \begin{matrix}{h_{11}(m)} \\{h_{21}(m)} \\{h_{31}(m)} \\{h_{41}(m)}\end{matrix} \right\rbrack + \begin{bmatrix}n_{11} \\n_{21} \\n_{31} \\n_{41}\end{bmatrix}}}}$where

-   η(m)=−Re(s(3(m−1)))+jIm(s(3(m−1)+1)),-   κ(m)=−Re(s(3(m−1)+1))+jImag(s(3(m−1))),-   ν(m)=Re(s(3(m−1)+1))+jIm(s(3(m−1))),-   ζ(m)=−Re(s(3(m−1)))−jImag(s(3(m−1)+1)),-   h_(kl)(m) is the frequency channel response of the channel between    transmitter k and receiver 1.    Similarly, the received data for the 4×2 configuration is    $\begin{bmatrix}    {y_{2}\left( {q,m} \right)} \\    {y_{2}\left( {{q + 1},m} \right)} \\    {y_{2}\left( {{q + 2},m} \right)} \\    {y_{2}\left( {{q + 3},m} \right)}    \end{bmatrix} = {\begin{bmatrix}    \left. {s\left( {{3m} - 3} \right)} \right) & {s\left( {{3m} - 2} \right)} & \frac{s\left( {{3m} - 1} \right)}{\sqrt{2}} & \frac{s\left( {{3m} - 1} \right)}{\sqrt{2}} \\    {- {s\left( {{3m} - 2} \right)}^{*}} & {s\left( {{3m} - 3} \right)}^{*} & \frac{s\left( {{3m} - 1} \right)}{\sqrt{2}} & {- \frac{s\left( {{3m} - 1} \right)}{\sqrt{2}}} \\    \frac{{s\left( {{3m} - 1} \right)}^{*}}{\sqrt{2}} & \frac{{s\left( {{3m} - 1} \right)}^{*}}{\sqrt{2}} & {\eta(m)} & {\kappa(m)} \\    \frac{{s\left( {{3m} - 1} \right)}^{*}}{\sqrt{2}} & {- \frac{{s\left( {{3m} - 1} \right)}^{*}}{\sqrt{2}}} & {\nu(m)} & {\zeta(m)}    \end{bmatrix}{\quad\;{\left\lbrack \begin{matrix}    {h_{12}(m)} \\    {h_{22}(m)} \\    {h_{32}(m)} \\    {h_{42}(m)}    \end{matrix} \right\rbrack + \begin{bmatrix}    n_{11} \\    n_{21} \\    n_{31} \\    n_{41}    \end{bmatrix}}}}$

The solution will be the least mean square solution by enumerating allpossibilities. Suppose instead that sub-carrier m is SM Coded, i.e. mbelongs to G2. For a 2×2 configuration, there are 4 equations and 4unknowns: $\begin{bmatrix}{y_{1}\left( {q,m} \right)} \\{y_{2}\left( {q,m} \right)}\end{bmatrix} = {{\begin{bmatrix}{h_{11}\left( {q,m} \right)} & {h_{21}\left( {q,m} \right)} \\{h_{12}\left( {q,m} \right)} & {h_{22}\left( {q,m} \right)}\end{bmatrix}\;\left\lbrack \begin{matrix}{s\left( {2m} \right)} \\{s\left( {{2m} + 1} \right)}\end{matrix} \right\rbrack} + {\quad{{\begin{bmatrix}{n_{1}\left( {q,m} \right)} \\{n_{1}\left( {q,m} \right)}\end{bmatrix}\begin{bmatrix}{y_{1}\left( {{q + 1},m} \right)} \\{y_{2}\left( {{q + 1},m} \right)}^{*}\end{bmatrix}} = {{\begin{bmatrix}{h_{11}\left( {q,m} \right)} & {h_{21}\left( {q,m} \right)} \\{h_{12}\left( {q,m} \right)} & {h_{22}\left( {q,m} \right)}\end{bmatrix}\;\left\lbrack \begin{matrix}{s\left( {{2m} + 2} \right)} \\{s\left( {{2m} + 3} \right)}\end{matrix} \right\rbrack} + {\quad\begin{bmatrix}{n_{2}\left( {{q + 1},m} \right)} \\{n_{2}\left( {{q + 1},m} \right)}\end{bmatrix}}}}}}$

So the 4 unknowns can be estimated by the least mean square solutions.

For a 2×3 configuration, there are 6 equations and 4 unknowns.$\begin{bmatrix}{y_{1}\left( {q,m} \right)} \\{y_{2}\left( {q,m} \right)} \\{y_{3}\left( {q,m} \right)}\end{bmatrix} = {{\begin{bmatrix}{h_{11}\left( {q,m} \right)} & {h_{21}\left( {q,m} \right)} \\{h_{12}\left( {q,m} \right)} & {h_{22}\left( {q,m} \right)} \\{h_{13}\left( {q,m} \right)} & {h_{23}\left( {q,m} \right)}\end{bmatrix}\;\left\lbrack \begin{matrix}{s\left( {2m} \right)} \\{s\left( {{2m} + 1} \right)}\end{matrix} \right\rbrack} + {\quad{{\begin{bmatrix}{n_{1}\left( {q,m} \right)} \\{n_{2}\left( {q,m} \right)} \\{n_{3}\left( {q,m} \right)}\end{bmatrix}\begin{bmatrix}{y_{1}\left( {{q + 1},m} \right)} \\{y_{2}\left( {{q + 1},m} \right)} \\{y_{3}\left( {{q + 1},m} \right)}\end{bmatrix}} = {{\begin{bmatrix}{h_{11}\left( {{q + 1},m} \right)} & {h_{21}\left( {{q + 1},m} \right)} \\{h_{12}\left( {{q + 1},m} \right)} & {h_{22}\left( {{q + 1},m} \right)} \\{h_{13}\left( {{q + 1},m} \right)} & {h_{23}\left( {{q + 1},m} \right)}\end{bmatrix}\;\left\lbrack \begin{matrix}{s\left( {{2m} + 2} \right)} \\{s\left( {{2m} + 3} \right)}\end{matrix} \right\rbrack} + {\quad\begin{bmatrix}{n_{1}\left( {{q + 1},m} \right)} \\{n_{2}\left( {{q + 1},m} \right)} \\{n_{3}\left( {{q + 1},m} \right)}\end{bmatrix}}}}}}$

For a 2×4 configuration, there are 8 equations and 4 unknowns

For a 3×3 configuration, there are 9 equations and 9 unknowns.

In accordance with the inventive architecture, the data rate can be ashigh as 70 Mbps for 2×2 and 120 Mbps for 3×3 within 6 MHz spectrum.

An exemplary optimal threshold value for a 2×2 configuration is 0.5. Anexemplary optimal threshold value for a 2×4 configuration is 1.0. Anexemplary optimal threshold value for a 3×3 configuration is 1.2. Anexemplary optimal threshold value for a 2×3 configuration is 1.0. Byexemplary optimal threshold value, the intent is to attain a value thathas a trade-off between time and spatial diversity that yields both arelatively high robustness and relatively high data packet ratetransfer.

As can be appreciated for each of the afore-mentioned configurations,there are a certain number of equations and a certain number ofunknowns. In an over-determined system, the number of equations isgreater than the number of unknowns. Thus, for a 2×2 configuration,there are two unknowns but four equations may be formulated. If there isno noise, any two of them (six pairs), or any three of them (fourtriples) or all of the four equations (one quadratic) will give the sameanswer. The difference is when noise is present, because thecombinations with then give different solutions. Since some of thesolutions may be good while others are bad, different combinations arechosen, but those combinations that result in large derivations are tobe avoided. The idea is to use a sub-set of the over determined linearequations to estimate the solution and then average all the possiblesolutions that seem viable. The averaging may be done with a least meansquare solution, which is a conventional mathematical technique.

FIG. 8 compares a two receiver antenna case and a three receiver antennacase. With respect to the three receiver antenna case, the number ofreceiver antennas is greater than the number of transmitter antennas. Asa consequence, the receiver has additional redundancy, the receiver hasvarious configurations, and the configurations yield several differentdecoding results. The most reliable solution can be selected from amongthem or all the solutions may be averaged to obtain a final result.

While the foregoing description and drawings represent the preferredembodiments of the present invention, it will be understood that variouschanges and modifications may be made without departing from the spiritand scope of the present invention.

1. An apparatus for use with an adaptive orthogonal frequencydivision-multiplexing (OFDM) system that uses multiple input multipleoutput (MIMO) structure to transmit OFDM signals from a plurality oftransmitter antennas to a plurality of receiver antennas, at least oneof the OFDM signals having an OFDM frame of a duration, the OFDM framehaving data packets and a plurality of OFDM slots, each of the OFDMslots having a plurality of OFDM symbols that include a plurality ofsub-carriers, the apparatus comprising: a receiver that responds toreceipt of the at least one OFDM signal by making a determination for asub-carrier of the plurality of sub-carriers as to whether timediversity or spatial diversity should be used for subsequenttransmission on the sub-carrier and transmitting a feedback signalindicative of that determination, wherein OFDM data signals that aretransmitted on the sub-carrier over multiple ones of the transmitterantennas are independent of each other for the spatial diversity andcorrespond to each other for the time diversity.
 2. An apparatus as inclaim 1, wherein the receiver includes a controller that makes thedetermination based on a comparison of a channel condition with athreshold, the channel condition being based on a frequency responsechannel matrix that is derived from OFDM symbols.
 3. An apparatus as inclaim 2, wherein the channel condition is based on a calculation of asmallest eigen value of the frequency response channel matrix.
 4. Anapparatus as in claim 2, wherein the channel condition is based on adetermination of a smallest element in a diagonal of the frequencyresponse channel matrix.
 5. An apparatus as in claim 2, wherein thechannel condition represents a ratio of largest and smallest eigenvalues of the channel matrix.
 6. An apparatus as in claim 2, wherein thechannel condition is based on one of three criteria selected from agroup consisting of a calculation of smallest eigen values of thechannel matrix, a smallest element in a diagonal of the channel matrix,and a ratio of largest and smallest eigen values of the channel matrix.7. An apparatus as in claim 2, further comprising a channel estimatorthat forms the frequency response channel matrix.
 8. An apparatus as inclaim 2, wherein the controller is configured to classify eachsub-carrier of the plurality of sub-carriers into one of two groups inaccordance with a respective channel condition for that sub-carrier, oneof the two groups being indicative of time diversity and the other ofthe two groups being indicative of spatial diversity.
 9. An apparatusfor use with an adaptive orthogonal frequency division-multiplexing(OFDM) system that uses multiple input multiple output (MIMO) structureto transmit OFDM signals from a plurality of transmitter antennas to aplurality of receiver antennas, at least one of the OFDM signals havingan OFDM frame of a duration, the OFDM frame having data packets and aplurality of OFDM slots, each of the OFDM slots having a plurality ofOFDM symbols that include a plurality of sub-carriers, the apparatuscomprising: at least one controller configured and arranged to respondto a feedback signal, the feedback signal indicative of a determinationfor a sub-carrier of the plurality of sub-carriers as to whether timediversity or spatial diversity should be used for subsequenttransmission on the sub-carrier, to direct an encoder to assignconstellation points for the time diversity or the spatial diversity tothe sub-carrier, the encoder including a space time transmitterdiversity (STTD) encoder and a spatial multiplexing (SM) encoder, theSTTD encoder being arranged to encode the sub-carrier in accordance withthe time diversity and the SM encoder being arranged to encode thesub-carrier in accordance with the spatial diversity, wherein OFDM datasignals that are transmitted on the sub-carrier over multiple ones ofthe transmitter antennas are independent of each other for the spatialdiversity and correspond to each other for the time diversity.
 10. Anapparatus as in claim 9, wherein the controller is configured todetermine a modulation scheme on each of the sub-carriers based on anestimated ratio selected from a further group consisting of a carrier tointerference ratio and a signal to noise ratio.
 11. An apparatus for usewith an adaptive orthogonal frequency division multiplexing (OFDM)system that uses multiple input multiple output (MIMO) structure totransmit OFDM signals from a plurality of transmitter antennas to aplurality of receiver antennas, at least one of the OFDM signals havingan OFDM frame of a duration, the OFDM frame having data packets and aplurality of OFDM slots, each of the OFDM slots having a plurality ofOFDM symbols that include a plurality of sub-carriers, the apparatuscomprising: controllers configured and arranged to direct transmissionand reception in accordance with OFDM, the controllers including thoseassociated with the reception that are configured to respond to receiptof the at least one OFDM signal by making a determination for asub-carrier of the plurality of sub-carriers as to whether timediversity or spatial diversity should be used for subsequenttransmission on the sub-carrier and transmitting a feedback signalindicative of that determination, wherein OFDM data signals that aretransmitted on the sub-carrier over multiple ones of the transmitterantennas are independent of each other for the spatial diversity andcorrespond to each other for the time diversity, the controllersincluding those associated with the transmission that are responsive toreceipt of the feedback signal to direct an encoder to assignconstellation points for either the time diversity or the spatialdiversity to the sub-carrier, the encoder including a space timetransmitter diversity (STTD) encoder and a spatial multiplexing (SM)encoder, the STTD encoder being arranged to encode the sub-carrier inaccordance with the time diversity and the SM encoder being arranged toencode the sub-carrier in accordance with the spatial diversity.
 12. Anapparatus as in claim 11, wherein the controllers are configured todetermine a modulation scheme on each of the sub-carriers based on anestimated ratio selected from a further group consisting of a carrier tointerference ratio and a signal to noise ratio.
 13. An apparatus as inclaim 12, wherein the controllers associated with the reception areconfigured to make a calculation of eigen values of channel matrices tomake a determination as to which sub-carriers are to use the timediversity during a subsequent transmission and which sub-carriers are touse the spatial diversity during the subsequent transmission, thecontrollers associated with the reception being configured to make thedetermination based on a comparison between a threshold and the eigenvalues and to direct transmission of a feed back signal indicative of aresult of the determination.
 14. An apparatus as in claim 11, whereinthe controllers associated with the reception are configured to make thedetermination based on a comparison of a channel condition with athreshold, the channel condition being based on a frequency responsechannel matrix that is derived from OFDM symbols.
 15. An apparatus as inclaim 14, wherein the channel condition represents a calculation of asmallest eigen value of the frequency response channel matrix.
 16. Anapparatus as in claim 14, wherein the channel condition represents adetermination of a smallest element in a diagonal of the frequencyresponse channel matrix.
 17. An apparatus as in claim 14, wherein thechannel condition represents a ratio of largest and smallest eigenvalues of the channel matrix.
 18. An apparatus as in claim 14, whereinthe channel condition represents one of three criteria selected from agroup consisting of a calculation of smallest eigen values of thechannel matrix, a smallest element in a diagonal of the channel matrix,and a ratio of largest and smallest eigen values of the channel matrix.19. An apparatus as in claim 14, further comprising a channel estimatorthat forms the frequency response channel matrix.
 20. A method for usewith an adaptive orthogonal frequency division-multiplexing (OFDM)system that uses multiple input multiple output (MIMO) structure totransmit OFDM signals from a plurality of transmitter antennas to aplurality of receiver antennas, at least one of the OFDM signals havingan OFDM frame of a duration, the OFDM frame having data packets and aplurality of OFDM slots, each of the OFDM slots having a plurality ofOFDM symbols that include a plurality of sub-carriers, the methodcomprising: responding to receipt of the OFDM signal by making adetermination for a sub-carrier of the plurality of sub-carriers as towhether time diversity or spatial diversity should be used forsubsequent transmission on the sub-carrier and transmitting a feedbacksignal indicative of that determination, wherein OFDM data signals thatare transmitted on the sub-carrier over multiple ones of the transmitterantennas are independent of each other for the spatial diversity andcorrespond to each other for the time diversity.
 21. A method as inclaim 20, further comprising making the determination based on acomparison of a channel condition with a threshold, the channelcondition being based on a frequency response channel matrix that isderived from OFDM symbols.
 22. A method as in claim 21, furthercomprising calculating a smallest eigen value of the frequency responsechannel matrix and basing the channel condition on the calculating. 23.A method as in claim 21, further comprising determining a smallestelement in a diagonal of the frequency response channel matrix andbasing the channel condition on the determining.
 24. A method as inclaim 21, further comprising calculating a ratio of largest and smallesteigen values of the channel matrix and basing the channel condition onthe ratio.
 25. A method as in claim 21, further comprising basing thechannel condition on one of three criteria selected from a groupconsisting of a calculation of smallest eigen values of the channelmatrix, a smallest element in a diagonal of the channel matrix, and aratio of largest and smallest eigen values of the channel matrix.
 26. Amethod as in claim 20, further comprising classifying each sub-carrierof the plurality of sub-carriers into one of two groups in accordancewith a respective channel condition for that sub-carrier, one of the twogroups being indicative of time diversity and the other of the twogroups being indicative of spatial diversity.
 27. A method for use withan adaptive orthogonal frequency division-multiplexing (OFDM) systemthat uses multiple input multiple output (MIMO) structure to transmitOFDM signals from a plurality of transmitter antennas to a plurality ofreceiver antennas, at least one of the OFDM signals having an OFDM frameof a duration, the OFDM frame having data packets and a plurality ofOFDM slots, each of the OFDM slots having a plurality of OFDM symbolsthat include a plurality of sub-carriers, the method comprising:responding to a feedback signal, the feedback signal indicative of adetermination for a sub-carrier of the plurality of sub-carriers as towhether time diversity or spatial diversity should be used forsubsequent transmission on the sub-carrier, to direct an encoder toassign constellation points for the time diversity or the spatialdiversity to the sub-carrier, the encoder including a space timetransmitter diversity (STTD) encoder and a spatial multiplexing (SM)encoder, the STTD encoder being arranged to encode the sub-carrier inaccordance with the time diversity and the SM encoder being arranged toencode the sub-carrier in accordance with the spatial diversity, whereinOFDM data signals that are transmitted on the sub-carrier over multipleones of the transmitter antennas are independent of each other for thespatial diversity and correspond to each other for the time diversity.28. A method as in claim 27, further comprising classifying eachsub-carrier of the plurality of sub-carriers into one of two groups inaccordance with a respective channel condition for that sub-carrier, oneof the two groups being indicative of time diversity and the other ofthe two groups being indicative of spatial diversity.
 29. A method foruse with an adaptive orthogonal frequency division-multiplexing (OFDM)system that uses multiple input multiple output (MIMO) structure totransmit OFDM signals from a plurality of transmitter antennas to aplurality of receiver antennas, at least one of the OFDM signals havingan OFDM frame of a duration, the OFDM frame having data packets and aplurality of OFDM slots, each of the OFDM slots having a plurality ofOFDM symbols that include a plurality of sub-carriers, the methodcomprising: directing transmission and reception in accordance with OFDMby using controllers, the controllers including those associated withthe reception responding to receipt of the at least one OFDM signal bymaking a determination for a sub-carrier of the plurality ofsub-carriers as to whether time diversity or spatial diversity should beused for subsequent transmission on the sub-carrier and transmitting afeedback signal indicative of that determination, wherein OFDM datasignals that are transmitted on the sub-carrier over multiple ones ofthe transmitter antennas are independent of each other for the spatialdiversity and correspond to each other for the time diversity, thecontrollers including those associated with the transmission thatrespond to receipt of the feedback signal to direct an encoder to assignconstellation points for the time diversity or the spatial diversity tothe sub-carrier, the encoder including a space time transmitterdiversity (STTD) encoder and a spatial multiplexing (SM) encoder, theSTTD encoder being arranged to encode the sub-carrier in accordance withthe time diversity and the SM encoder being arranged to encode thesub-carrier in accordance with the spatial diversity.
 30. A method as inclaim 29, wherein the controllers associated with the reception make acalculation of eigen values of channel matrices to make a determinationas to which sub-carriers are to use the time diversity during asubsequent transmission and which sub-carriers are to use the spatialdiversity during the subsequent transmission, the controllers associatedwith the reception make the determination based on a comparison betweena threshold and the eigen values and direct transmission of a feed backsignal indicative of a result of the determination.
 31. A method as inclaim 29, wherein the controllers associated with the reception make thedetermination based on a comparison of a channel condition with athreshold, the channel condition being based on a frequency responsechannel matrix that is derived from OFDM symbols.
 32. A method as inclaim 31, further comprising calculating a smallest eigen value of thefrequency response channel matrix basing the channel condition on thecalculating.
 33. A method as in claim 31, further comprising determininga smallest element in a diagonal of the frequency response channelmatrix and basing the channel condition on the determining.
 34. A methodas in claim 31, further comprising calculating a ratio of largest andsmallest eigen values of the channel matrix and basing the channelcondition on the ratio.
 35. A method as in claim 31, further comprisingbasing the channel condition on one of three criteria selected from agroup consisting of a calculation of smallest eigen values of thechannel matrix, a smallest element in a diagonal of the channel matrix,and a ratio of largest and smallest eigen values of the channel matrix.36. A method as in claim 29, further comprising determining a modulationscheme on each of the sub-carriers based on an estimated ratio selectedfrom a further group consisting of a carrier to interference ratio and asignal to noise ratio.
 37. An apparatus as in claim 2, wherein thecontroller is further configured to determine a modulation scheme oneach of the plurality of sub-carriers based on an estimated ratioselected from a group consisting of a carrier to interference ratio anda signal to noise ratio.
 38. A method as in claim 20, further comprisingdetermining a modulation scheme on each of the plurality of sub-carriersbased on an estimated ratio selected from a group consisting of carrierto interference ratio and signal to noise ratio.
 39. A method as inclaim 27, further comprising determining a modulation scheme on each ofthe plurality of sub-carriers based on an estimated ratio selected froma group consisting of a carrier to interference ratio and a signal tonoise ratio.
 40. An apparatus as in claim 1, wherein the subsequenttransmission comprises transmission units comprising M OFDM symbols,where M is the number of transmitter antennas in the OFDM system.
 41. Anapparatus as in claim 9, wherein the subsequent transmission comprisestransmission units comprising M OFDM symbols, where M is the number oftransmitter antennas in the OFDM system.
 42. An apparatus as in claim11, wherein the subsequent transmission comprises transmission unitscomprising M OFDM symbols, where M is the number of transmitter antennasin the OFDM system.
 43. A method as in claim 20, further comprisingusing transmission units comprising M OFDM symbols for the subsequenttransmission, where M is the number of transmitter antennas in the OFDMsystem.
 44. A method as in claim 27, further comprising usingtransmission units comprising M OFDM symbols for the subsequenttransmission, where M is the number of transmitter antennas in the OFDMsystem.
 45. A method as in claim 29, further comprising usingtransmission units comprising M OFDM symbols for the subsequenttransmission, where M is the number of transmitter antennas in the OFDMsystem.
 46. An apparatus as in claim 1, wherein the receiver makes acalculation of eigen values of a plurality of channel matrices to make adetermination as to which sub-carriers are to use the time diversityduring a subsequent transmission and which sub-carriers are to use thespatial diversity during the subsequent transmission, the receiver beingconfigured to make the determination based on a comparison between athreshold and the eigen values and to direct transmission of a feed backsignal indicative of a result of the determination.
 47. A method as inclaim 20, further comprising calculating eigen values of a plurality ofchannel matrices to make a determination as to which sub-carriers are touse the time diversity during a subsequent transmission and whichsub-carriers are to use the spatial diversity during the subsequenttransmission, wherein the determination is made based on a comparisonbetween a threshold and the eigen values, and transmitting a feed backsignal indicative of a result of the determination.
 48. An apparatus asin claim 1, wherein the receiver responds to receipt of the at least oneOFDM signal by designating each of the plurality of sub-carriers as anelement of one of two sets, wherein one set is for sub-carriers thatshould use time diversity for subsequent transmission and the other setis for sub-carriers that should use spatial diversity for subsequenttransmission, and transmitting the set with fewer elements as thefeedback signal.
 49. An apparatus as in claim 9, wherein the feedbacksignal indicates either a set of sub-carriers of the plurality ofsub-carriers that should use time diversity for subsequent transmissionor a set of sub-carriers of the plurality of sub-carriers that shoulduse spatial diversity for subsequent transmission depending on which sethas fewer elements.
 50. An apparatus as in claim 11, wherein thecontrollers associated with the reception respond to receipt of the atleast one OFDM signal by designating each of the plurality ofsub-carriers as an element of one of two sets, wherein one set is forsub-carriers that should use time diversity for subsequent transmissionand the other set is for sub-carriers that should use spatial diversityfor subsequent transmission, and transmitting the set with fewerelements as the feedback signal.
 51. A method as in claim 20, whereinresponding to receipt of the at least one OFDM signal comprisesdesignating each of the plurality of sub-carriers as an element of oneof two sets, wherein one set is for sub-carriers that should use timediversity for subsequent transmission and the other set is forsub-carriers that should use spatial diversity for subsequenttransmission, and transmitting the set with fewer elements as thefeedback signal.
 52. A method as in claim 27, wherein the feedbacksignal indicates either a set of sub-carriers of the plurality ofsub-carriers that should use time diversity for subsequent transmissionor a set of sub-carriers of the plurality of sub-carriers that shoulduse spatial diversity for subsequent transmission depending on which sethas fewer elements.
 53. A method as in claim 29, wherein responding toreceipt of the at least one OFDM signal comprises designating each ofthe plurality of sub-carriers as an element of one of two sets, whereinone set is for sub-carriers that should use time diversity forsubsequent transmission and the other set is for sub-carriers thatshould use spatial diversity for subsequent transmission, andtransmitting the set with fewer elements as the feedback signal.
 54. Anapparatus as in claim 1, wherein the receiver responds to receipt of theOFDM signal by making a determination for a subset of the plurality ofsub-carriers as to whether time diversity or spatial diversity should beused for subsequent transmission on the subset of the sub-carriers. 55.An apparatus as in claim 9, wherein at least one controller isconfigured and arranged to respond to a feedback signal, the feedbacksignal indicative of a determination for a subset of the plurality ofsub-carriers as to whether time diversity or spatial diversity should beused for subsequent transmission on the subset of the sub-carriers, todirect an encoder to assign constellation points for the time diversityor the spatial diversity to the subset of the sub-carriers.
 56. Anapparatus as in claim 11, wherein the controllers including thoseassociated with the reception that are configured to respond to receiptof the at least one OFDM signal by making a determination for a subsetof the plurality of sub-carriers as to whether time diversity or spatialdiversity should be used for subsequent transmission on the subset ofthe sub-carriers.
 57. A method as in claim 20, wherein the respondingcomprises responding to receipt of the OFDM signal by making adetermination for a subset of the plurality of sub-carriers as towhether time diversity or spatial diversity should be used forsubsequent transmission on the sub-carrier.
 58. A method as in claim 27,wherein the responding comprises responding to a feedback signal, thefeedback signal indicative of a determination for a subset of theplurality of sub-carriers as to whether time diversity or spatialdiversity should be used for subsequent transmission on the subset ofthe sub-carriers, to direct an encoder to assign constellation pointsfor the time diversity or the spatial diversity to the subset of thesub-carriers.
 59. A method as in claim 29, wherein the controllersincluding those associated with the reception respond to receipt of theat least one OFDM signal by making a determination for a subset of theplurality of sub-carriers as to whether time diversity or spatialdiversity should be used for subsequent transmission on the subset ofthe sub-carriers.
 60. An apparatus as in claim 1, wherein the receiverresponds to receipt of the OFDM signal by making a determination forsub-carriers of an OFDM symbol from the plurality of OFDM symbols as towhether time diversity or spatial diversity should be used forsubsequent transmission on the sub-carriers of the OFDM symbol.
 61. Anapparatus as in claim 9, wherein at least one controller is configuredand arranged to respond to a feedback signal, the feedback signalindicative of a determination for sub-carriers of an OFDM symbol fromthe plurality of OFDM symbols as to whether time diversity or spatialdiversity should be used for subsequent transmission on the sub-carriersof the OFDM symbol, to direct an encoder to assign constellation pointsfor the time diversity or the spatial diversity to the sub-carriers ofthe OFDM symbol.
 62. An apparatus as in claim 11, wherein thecontrollers including those associated with the reception that areconfigured to respond to receipt of the at least one OFDM signal bymaking a determination for sub-carriers of an OFDM symbol from theplurality of OFDM symbols as to whether time diversity or spatialdiversity should be used for subsequent transmission on the sub-carriersof the OFDM symbol.
 63. A method as in claim 20, wherein the respondingcomprises responding to receipt of the OFDM signal by making adetermination for sub-carriers of an OFDM symbol from the plurality ofOFDM symbols as to whether time diversity or spatial diversity should beused for subsequent transmission on the sub-carrier.
 64. A method as inclaim 27, wherein the responding comprises responding to a feedbacksignal, the feedback signal indicative of a determination forsub-carriers of an OFDM symbol from the plurality of OFDM symbols as towhether time diversity or spatial diversity should be used forsubsequent transmission on the sub-carriers of the OFDM symbol, todirect an encoder to assign constellation points for the time diversityor the spatial diversity to the sub-carriers of the OFDM symbol.
 65. Amethod as in claim 29, wherein the controllers including thoseassociated with the reception respond to receipt of the at least oneOFDM signal by making a determination for sub-carriers of an OFDM symbolfrom the plurality of OFDM symbols as to whether time diversity orspatial diversity should be used for subsequent transmission on thesub-carriers of the OFDM symbol.
 66. A method for use with an adaptiveorthogonal frequency division-multiplexing (OFDM) system that usesmultiple input multiple output (MIMO) structure to transmit OFDM signalsfrom a plurality of transmitter antennas to a plurality of receiverantennas, at least one of the OFDM signals having an OFDM frame of aduration, the OFDM frame having data packets and a plurality of OFDMslots, each of the OFDM slots having a plurality of OFDM symbols thatinclude a plurality of sub-carriers, the method comprising: respondingto receipt of the OFDM signal by making a determination as to whethertime diversity or spatial diversity should be used for subsequenttransmission and transmitting a feedback signal indicative of thatdetermination, wherein OFDM data signals that are transmitted overmultiple ones of the transmitter antennas are independent of each otherfor the spatial diversity and correspond to each other for the timediversity and wherein the determination is based on a channel conditionindicated by a criteria selected from the group of criteria consistingof an eigen value of a channel matrix, an element in a diagonal of thechannel matrix and a plurality of eigen values of the channel matrix.67. A method for use with an adaptive orthogonal frequencydivision-multiplexing (OFDM) system that uses multiple input multipleoutput (MIMO) structure to transmit OFDM signals from a plurality oftransmitter antennas to a plurality of receiver antennas, at least oneof the OFDM signals having an OFDM frame of a duration, the OFDM framehaving data packets and a plurality of OFDM slots, each of the OFDMslots having a plurality of OFDM symbols that include a plurality ofsub-carriers, the method comprising: responding to a feedback signal todirect an encoder to assign constellation points for time diversity orspatial diversity to at least one sub-carrier of the plurality ofsub-carriers in accordance with a channel condition, the encoderincluding a space time transmitter diversity (STTD) encoder and aspatial multiplexing (SM) encoder, the STTD encoder being arranged toencode the at least one sub-carrier in accordance with time diversityand the SM encoder being arranged to encode the at least one sub-carrierin accordance with spatial diversity, wherein OFDM data signals that aretransmitted over multiple ones of the transmitter antennas areindependent of each other for the spatial diversity and correspond toeach other for the time diversity and wherein the channel condition isdetermined based on a criteria selected from the group of criteriaconsisting of an eigen value of a channel matrix, an element in adiagonal of the channel matrix and a plurality of eigen values of thechannel matrix.